From elsewhere:
The ethyl derivatives of decaborane, made by Friedel-Crafts reaction of decaborane with ethyl bromide, was found to be easy to store and handle and was code named HEF-3 (High Energy Fuel-3). This was considered for use as fuel in the Mach 3 B-70 Valkyrie. But the programme was cancelled after spending millions of dollars. One reason was that borane-based fuels are highly corrosive and could spontaneously ignite. Also, when burned they produced solid combustion by-products, especially hard boron carbide and boron oxides, that became attached to the engine parts leading to mechanical faults and corrosion. In addition the sooty exhaust was highly toxic and has high visibility, making it unfit for defence use. Adding to this were the exorbitant cost of making such fuels and hence the borane-based fuel programme was abandoned.